diag: dci: Validate pkt length before parsing for full header

Few commands with smaller length than dci packet request header
can fail due to present header length check. Modify the length
check to cater to smaller length packets.

Change-Id: Icf2e45b4eb1be0f2a15f47e58baffe86ece20a1d
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>